How can I generate natural language sentences given logical structures that contain the subject, verb and target?
It might be simpler to generate the language output yourself since you've already got a concrete concept structure in code.

Maybe you'll also want to be aware of potential future applications such as parsing your own output back to the inputs.

With that said, you could investigate Markov Chains or Google around a bit for Natural Language Processing and Natural Language Generation.

https://stackoverflow.com/questions/33068943/library-for-generating-natural-language-verbs-in-javascript